Status and Trends of Resources Below Glen Canyon Dam Update 2009

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Scientists with the USGS Grand Canyon Monitor- ing and Research Center collecting water samples during the 2008 high -flow experiment (USGS photo by Paul Alley). the early life- history dynamics of rainbow trout in Lees Ferry showed that hourly flow fluctuations do affect the neashore habitat use and growth of young trout (less than 1 year old). For example, nearshore catch rates increased two to four times at the daily minimum flow compared to the daily maximum The effects of winter experimen- tal fluctuating flows of 2003 -2005, flows designed to disrupt spawning activity to reduce trout numbers and increase their size, were also evaluated for Lees Ferry. Survival rates increased as egg deposition decreased, meaning that incubation mortality rates owing to experimental flows were insuf- ficient to reduce the numbers of young fish. In 2006, for example, early survival rates posted a six -fold increase despite a ten -fold decrease in egg deposition. In early 2003, a major effort was begun by the GCDAMP to remove nonnative fish, particularly rainbow and brown trout (Salmo trutta), from the Colorado River near the confluence of the Little Colroado River, which is considered important habitat for the humpback chub. Not only do trout rely on the same food sources — aquatic and terrestrial invertebrates, algae, and small fish - -trout are also thought to prey on juvenile humpback chub. Between 2003 and 2006 the rainbow trout population in the Colorado River near the Little Colorado River was reduced by more than 80 %. Riparian Vegetation Historically, the flow of the Colorado River varied greatly by season, swelling eoWO a ;n7p;sgnS saa.tnosam uetunu ro uofsiAlln with snowmelt in the spring and slowing to a relative trickle by winter. Today, the regula- tion of the river by Glen Canyon Dam has reduced disturbance to vegetation caused by predam floods, and MLFF operations pro- vide a constant source of water for riparian plants (plants living near a waterway). These conditions have resulted in the expansion of woody and herbaceous vegetation, both native and nonnative species, immediately along the river channel. A limited study of vegetation change between 1992 and 2002 indicated that veg- etation expansion appears to be the greatest along shorelines. Additionally, monitoring of existing vegetation showed an increase in diameter, suggesting that individual woody plants are increasing in size. Increases in both the number and size of riparian plants may contribute to campsite area loss. Recent vegetation mapping efforts indicate that tamarisk (Tamarix ramosissima) is the dominant nonnative species, representing 24% of the vegetation community. The 2008 high -flow experiment was timed for early spring to decrease the likelihood of spread- ing tamarisk seeds, which would result in increased tamarisk colonization. Riparin habitat for the endangered Kanab ambersnail has increased since 1998; habi- tat is used as a surrogate for snail numbers because they fluctuate widely by season. The snail, which is associated with wetland and spring vegetation, is currently found at three locations, including two in Grand Canyon National Park (Vaseys Paradise and Elves Chasm). Vaseys Paradise is a small patch of spring -fed vegetation that is sur- veyed by the USGS in cooperation with the Arizona Game and Fish Department. Archeological Sites Grand Canyon has been used by humans for at least 13,000 years. Today, more than nine contemporary Native American tribes have cultural ties to the area. Grand Canyon National Park contains more than 2,600 doc- umented prehistoric sites; 336 sites are within the area potentially affected by Glen Canyon Dam operations. Cultural resource monitoring suggests that archeological resources are affected both by erosion and recreational visitors. Natural erosion patterns would happen whether the dam existed or not; however, the dam and its operations have limited the sediment available within the river corridor. The diminished sediment supply appears to be exacerbating the rate and amount of ero- sion affecting cultural resources. Sandbars created by a 2004 high -flow ex- perimental release from Glen Canyon Dam increased the windbome transport of new river deposited sand toward some archeologi- cal sites found near the Colorado River in Grand Canyon. Increasing the availability of sand that can be transported by the wind to ar- cheological sites in the river corridor, follow- ing sandbar building high flows (as described above) may reduce erosion and improve the condition of some archeological sites. Camping Beaches Camping area above the maximum water level permitted under MLFF dam opera- tions (the water level at a flow rate of 25,000 cubic feet per second) decreased by 55% between 1998 and 2003, which is an average annual decline of 15% per year. Relative to total sandbar area, area suitable for camping continues to decline, indicating that factors other than sandbar erosion—particularly vegetation encroachment — contribute to loss of campable area. Conclusion The regulated Colorado River below Glen Canyon Dam is a dynamic system affected by a range of factors such as dam operations and other conditions, including drought. The timely and high -quality scientific monitoring and research provided by USGS scientists and their cooperators about the effects of dam operations and other natural and human- caused actions on downstream resources provides information essential to effective adaptive management of Colorado River re- sources below Glen Canyon Dam. John F. Hamill For more information contact: U.S. Geological Survey Southwest Biological Science Center Grand Caynon Monitoring and Research Center Flagstaff, AZ 86001 928- 556 -7094 This Fact Sheet and any updates to it are available online at http : //pubs.usgs.gov/fs/2009/3033/ J31JJSIQ 100pS ltaiIeA aap1hog .`